just a thought (hard to be constructive without ability to compile or see bootlog messages): since the wifi is according to https://fortiblog.gitbook.io/fortinet/useful-information/fortigate-hardware-specification
WiFi Chipset: Atheros
WiFi firmware version: 0.9.17.1
do you have the right firmware and/or compile options... i'm thinking along the lines of what i see in [Solved] Setting up a Qualcomm Atheros QCA6174 adapter
good luck